Here is the link, it is a product from LDG.
http://www.ldgelectronics.com/products.php?cID=7&pID=2&v=1
73, Doug
N9XTF
Quoting k7fm <k7fm@teleport.com>:
> Hello Scott:
>
> Congratulations on the 813 amp. I had one many years ago and worked about
> 150 countries on 75 meters. The tubes are instant on - and that is worth a
> couple of db when you hear a rare station.
>
> The electronics of a tuned input is relatively easy - you simply need to
> match the impedance of the tubes to 50 ohms. Unless the amplifier you took
> the input from used the same tubes, it would not match, so you might as well
>
> build your own.
>
> The most difficult problem in retrofitting a tuned input to an existing
> amplifier is trying to figure out a way to switch the input with the output
> switch. I have done that successfully on a couple of occasions and it
> involves skill and good luck. So, plan on having a separate switch for the
> tuned input and remember to switch it with the output.
>
> If you wish to reduce designing to a minimum, get a pi-network antenna tuner
>
> and connect it right at the input - with zero length cable. Then adjust the
>
> match for the bands you want to operate on. You can then simply dissect the
>
> antenna tuner and determine what value parts you had when you had the best
> input swr. Then replace those measured or calculated values with mica caps
> and small coils. You can use a two wafer rotary switch to select the band.
>
> Or, you can design your own. I am guessing that the input impedance of an
> 813 is about 375 ohms. Two in parallel would be 188 ohms. Use one of the
> filter design programs to design a pi type filter that has 50 ohms input and
>
> 188 ohm output. Wind a toroid for the inductor and use mica compression
> trimmers for the capacitors and you should be able to hit it right on the
> money.
>
> On my 813 amp, I wound a broadband toroid step-up transformer and loaded the
>
> output with a bit of resistance to make the load constant and called it
> good. The driver was happy and I did not have to switch the tuned input. I
>
> did not have much equipment to measure the performance, but never got death
> threats from anyone so assumed it was ok.
>
> 73, Colin K7FM
>
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